A semiconductor device, such as DRAM, SRAM, Flash Memory, Optical Semiconductor Chip, etc., is fabricated through processes of forming a semiconductor layered film on a semiconductor substrate, perforating respective portions through etching, and filling with a metal film etc., so as to realize electrical connections, and further through processes of disposing trenches surrounding prescribed regions and filling with an oxide film so as to realize insulation. In the process of etching the layered film to form holes or trenches, various etching methods can be used, but plasma processing can be used to achieve deep and steep etching with a high aspect ratio, and therefore has been widely adopted for fabrication of most semiconductor devices.
Plasma is generated as follows: a gas for generation of plasmas is injected into an evacuated plasma generation chamber; and high frequency power (generally RF, microwave energy, etc.) is supplied into the plasma generation chamber, thereby forming a high frequency electrical field and generating plasma in the evacuated chamber. In order to precisely control the formation of the semiconductor circuit features, the plasma chamber should be designed so as to properly follow the control instruction. For example, the supply of high frequency power into a plasma processing chamber for generation, stabilization, maintenance, and extinguishing the plasma needs to be carefully controlled. Similarly, generating different levels of plasma densities, from a low density to a high density, shall be carried out with good controllability. Further, plasma processing is also used in formation and/or deposition of various films of the semiconductor device. The controllability of supplying the plasma processing chamber with high frequency power becomes a crucial factor in terms of controlling the characteristics of the formed films.
For the control of supplying the plasma processing chamber with high frequency power, the following control approaches are adopted in a high frequency power supply device. (1) One is an approach in which power is applied to the plasma chamber in the form of incident wave (incident or traveling wave high frequency power Pf). A reflected wave (reflected high frequency power Pr) is detected and fed back to a power amplifier, that is, a method in which a directivity coupler is used to separate the reflected wave (as returned from the plasma processing device) from the incident wave, and detect and feed back the reflected wave to the power amplifier. This is an approach of controlling the high frequency power itself. (2) The other is an approach of using an impedance matcher to achieve matching with the supplied high frequency power. The impedance matcher includes a detection block, which detects a phase difference φ and an impedance Z of a voltage and a current of the high frequency power, an impedance match block consisted of a capacitor C and an inductor L, a servo motor control block, which sets the phase difference detected by the detection block to zero, and automatically adjusts the capacitor C and the inductor L in such a way that a ratio of the voltage to the current becomes the characteristic impedance of a transmission line. The servo motor control block can also use a scale disc controlled manually. A method of using the impedance matcher is a method of controlling an efficiency with which the supplied high frequency power is used effectively for generation of plasma. With the approaches (1) and (2), a control on the supply of high frequency power used for generation of plasma can thus be achieved.
Unfortunately, the following problems are present with respect to the control by means of impedance matching in the approach (2). In the plasma processing device, a load impedance changes sharply before vs. after ignition of the plasma. Consequently, even if the servo motor mechanism together with the impedance matcher is used to achieve matching, various unfavorable situations may occur due to an insufficient follow-up speed. That is, due to inertia of the servo motor, etc., there may be a limitation of shortening the adjustment period, i.e., the response time of the impedance matching, which results in the occurrence of a case in which plasmas can not be generated quickly and stably. Further, a problem of intermediate extinguishing of plasma after it has been already ignited may also occur. In order to address the problems, such a method has been proposed in which the oscillating frequency of a high frequency oscillating block is variable, and additionally a plasma generation detector is disposed to detect the ignition of plasma inside the plasma processing chamber. Upon detection of generation of plasma by the plasma generation detector, the oscillating frequency of the high frequency oscillating block is set to a predetermined fixed frequency at which the plasmas is excited during generation of the plasma. Prior to generation of plasmas, a method can be adopted, in which the phase difference signal is received from the phase difference detector of the impedance matching block, and the oscillating frequency of the high frequency oscillating block is varied to make the phase difference zero. Dependent upon this method, generally referred to in the art as frequency tuning, plasma, after being generated, can be adjusted electrically directly to an optimum fixed frequency, and be supplied with high frequency, and thus can go into a stable status. That is, once the plasma has been ignited it is maintained by a fixed frequency RF source, and thus the impedance matcher can be used to achieve matching since no rapid impedance variations are expected after plasma ignition.
Further, the following problems are present in the approach (1) of feeding back the reflected wave to the high frequency amplifier. In many modern plasma chambers two RF frequencies are used; namely one high frequency for striking the plasma and controlling its ion density (generally referred to as source frequency), and a second, generally lower RF frequency, for controlling the energy of the ions in the plasma (generally referred to as bias frequency). The two RF frequencies are often applied to the same electrodes of the plasma processing chamber in an overlap manner. Consequently, the high frequency signal of the source frequency mixes with the reflected wave of the lower bias frequency. As a result, the reflected waves from the plasma processing chamber are thus formed of a spectrum of frequencies, including frequency-modulated waves and high order harmonics. That is, such a spectrum results, which includes side peaks only deviating a few wavelengths from the frequency of the bias frequency and centering on the source frequency signal. These peaks cannot be separated and detected, and thus become a crucial factor in terms of an error control of the power amplifier. Consequently, the reflected energy cannot be used for accurate frequency tuning.
In order to address these problems, that is, to resolve frequency mixing, such a method has been proposed in which a high frequency at a prescribed frequency is used for heterodyne detection, and this prescribed frequency is a frequency below the source frequency but above the frequency of high frequency power for controlling plasmas. According to this heterodyne detection method, such a spectrum can be generated around the high frequency signal of the above prescribed frequency, which includes side peaks that can be separated only by an amount of the frequency of the high frequency power for controlling ions, and the filter for selecting high frequency signal with desired frequency from the spectrum can be realized in a simple configuration. With this heterodyne detection method, a high frequency signal of the reflected wave can be captured and output to the power amplifier without error.
For further information the reader is directed to review Japanese laid-open publication No. Hei 9-161994 and Japanese laid-open publication No. 2003-179030.